In some wireless communication systems, when data that is transmitted by a transmitting entity to a receiving entity is not received by the receiving entity, or is received by the receiving entity with one or more errors, the data may be re-transmitted. The re-transmission of data could occur either automatically or in response to feedback from the receiving entity. For example, in Long Term Evolution (LTE) air interfaces, a Hybrid Automatic Repeat Request (HARQ) procedure is used. In the HARQ approach, after a transmitting entity has transmitted a block of data, the transmitting entity waits to receive a HARQ response from the receiving entity. If the transmitting entity receives a positive acknowledgement (ACK) as the HARQ response, then no re-transmission is needed and the transmitting entity can transmit additional data. If the transmitting entity receives a negative acknowledgement (NACK) as the HARQ response, then the transmitting entity re-transmits the data. The transmitting entity may also re-transmit the data if the transmitting entity does not receive any HARQ response within a certain period of time.
This re-transmission approach can allow data to be successfully transmitted from a transmitting entity to a receiving entity even when there is a substantial probability that the transmitted data will be received with one or more errors, for example, because of poor radio frequency (RF) conditions. Specifically, the data can be re-transmitted multiple times until the data is received without errors. This re-transmission approach, however, also increases latency. For example, there can be a period of delay between when the transmitting entity transmits data and when the transmitting data receives a NACK response from the receiving entity and another period of delay between when the transmitting entity receives the NACK response and when the transmitting entity begins re-transmitting the data.
In order to reduce the delay associated with re-transmitting data, LTE supports a bundling option for data transmissions by the user equipment (UE) in the Physical Uplink Shared Channel (PUSCH). Normally, the UE transmits data in one transmission time interval (TTI), which corresponds to a 1 millisecond (ms) subframe, and then waits to receive a HARQ response before re-transmitting the data or transmitting additional data. However, when TTI bundling is used, the UE transmits the same data four times in four consecutive TTIs and then waits to receive a HARQ response. In this way, the UE can transmit four instances of the same data, which allows for more robust reception of the data, but without the delay that would be associated with the UE transmitting the data four times and waiting for a HARQ response after each transmission.